Brokkos, Apex of Forever

• WOTC Official Reminder Textofficial rules
  Mutate — If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.

• WWizards of the Coast2020-04-17
  Official Ruling
  If you cast Brokkos with the permission granted by its last ability, you must pay its mutate cost to cast it. You can't choose to pay its mana cost or another alternative cost.
• WWizards of the Coast2020-04-17
  Official Ruling
  If another effect gives you permission to cast Brokkos from your graveyard, you may cast it for any cost associated with that permission.
• WWizards of the Coast2020-04-17
  Official Ruling
  Casting Brokkos with the permission granted by its last ability doesn't change when you can cast it.

Rulings via Scryfall